Danfoss Calculation standards for safety relief valves User guide
Calculation standards for
safety relief valves
In Coolselector®2
Niels P. Vestergaard & Morten Juel Skovrup
2019-02-21
Version 1.00
1 Introduction
Safety valves can be calculated and selected in Coolselector®2 according to either EN 13136 or ISO 5149.
In general, EN 13136 describes a more complete calculation and selection method than ISO 5149. Whether
to use one or the other, is often a matter of local legislation or customer requirements, but it is in any case
important to understand the differences between the two standards.
The page below describe the most important differences between the standards, but you should refer to
the latest version of the standards for a complete overview.
Calculation standards for safety relief valves
2
2 EN 13136
Calculation of required valve capacity
Vessel heat load: Assumes a constant heat flow acting on the total external surface of the
vessel. The heat of evaporation is dependent on the actual refrigerant
and pressure.
Compressor overflow valve: Included in EN 13136.
Trapped liquid: Included in EN 13136.
Calculation of selected valve capacity
Based on valve characteristics (rated flow area & discharge coefficient) and calculated flow with actual
refrigerant and refrigerant properties.
Calculation of inlet and outlet pipe lines
Inlet pressure drop calculation: Required (maximum allowed pressure drop is 3% of relief pressure for
standard safety valves). Changeover valve is included in calculation.
Outlet pressure drop calculation: Required (maximum allowed pressure drop is 10% of relief pressure for
standard safety valves).
3 ISO 5149
Calculation of required valve capacity
Vessel heat load: Assumes a constant heat flow acting proportional to the shell area of the
vessel (not including end caps). Using the area and a factor f depending
on the refrigerant, the corresponding required mass flow of air through
the safety relief valve can be calculated. The factor f is constant in the
whole application range.
Compressor overflow valve: Not included in ISO 5149 (but EN 13136 is mentioned as possible
calculation method in notes).
Trapped liquid: Not included in ISO 5149 (but EN 13136 is mentioned as possible
calculation method in notes).
Calculation of selected valve capacity
Rated air capacity stamped on the valve (in SCFM or kg/s).
Calculation of inlet and outlet pipe lines
Inlet pressure drop calculation: Not included in ISO 5149 (neither pipes and fittings nor changeover
valve).
Outlet pressure drop calculation: Required. But described procedure is not consistent in current version of
the standard (ISO 5149-2:2014), and calculation is therefore not
included in Coolselector®2.
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